11 



the left, however, is slightly more convex than the right 

 (see Text-fig. 1). Both species have siiborbicular valves, 

 and these are marked by plications so that the outer 

 surface has a number of ribs arising near the umbo. The 

 ribs are not present in old specimens of P. maximus on 

 the areas immediately adjoining the iimbos. 



Pig. i. Diagrammatic sections of P. opercularis and P. maximus to show- 

 shape of valves in natural position. 



The number of plications appears to be constant 

 throughout life, no new ones arising by bifurcation or 

 interposition, and there are fewer in P. maximus than in 

 P. opercularU. The average numbers can be obtained 

 from the following table given by Davenport (41 ) for the 

 shells of P . opercularis from three localities : — 



536 614 508 



Davenport has also given the relation of the dorso- 

 ventral diameter to the antero-posterior diameter, for 

 1,657 shells of P. opercularis from these same localities. 

 The results show that the smallest shells are from oft the 

 Eddystone lighthouse, the largest from the Firth of Forth, 

 and the intermediate ones from Port Erin in the Irish Sea. 



